1. Technical Field
The present disclosure relates to an optical device.
2. Description of the Related Art
In light measurement, light in a polarized state is, in some cases, converted into light in an unpolarized state, i.e., light whose polarized state is cancelled. The light in the polarized state changes to the light in the unpolarized state in such a manner that any of factors including a time, a location, and a wavelength is changed.
For example, the diffraction efficiency of a diffraction grating used for a spectrometer changes depending on the relationship between the direction of a groove of the diffraction grating and the polarization state of incident light.
In order to accurately measure a wavelength level regardless of the polarization state of incident light, a depolarization plate is, as described in, e.g., JP-A-2002-365592, provided at an input portion of a spectrometer to eliminate the influence of the difference based on the relationship between the direction of the groove and the polarization state of incident light.
FIG. 7 is a diagram for describing the configuration of an example of a typical spectrometer (monochromator) described in JP-A-2002-365592. In this spectrometer, a depolarization plate is provided at an input portion. As illustrated in FIG. 7, an incident light beam 1 enters a depolarization plate 3 through an input slit 2. An optical path is shown by only one line in FIG. 7. However, the light is actually split into two beams by the depolarization plate 3.
Two light beams split by the depolarization plate 3 are converted into parallel light beams by a first concave mirror 4. Then, such light beams enter a plane diffraction grating 5, and then, are diffracted.
The light beams diffracted by the plane diffraction grating 5 are condensed at a second concave mirror 6, and then, are output through an output slit 7.
FIG. 8 is a diagram for describing the configuration of the depolarization plate 3. The depolarization plate 3 illustrated in FIG. 7 includes two wedge plates 3a, 3b, for example. Two wedge plates 3a, 3b are bonded together such that the crystal optical axes thereof are perpendicular to each other. Two wedge plates 3a, 3b are formed of a birefringent material such as crystal plates. Two wedge plates 3a, 3b are cut such that the thickness thereof varies according to a predetermined shape.
In the depolarization plate 3 illustrated in FIG. 8, an ordinary beam of the wedge plate 3a is an extraordinary beam of the wedge plate 3b. An extraordinary beam of the wedge plate 3a is an ordinary beam of the wedge plate 3b. Thus, due to the difference in refractive index of the material, refraction occurs at the depolarization plate 3. Due to the difference in refraction direction, division of light occurs in the Y axis direction (in the direction of a groove of a plane diffraction grating 5 described later). The split light satisfies the following relationship:α=2(ne−no)tan θ0  (1)
where α: the angle between two split light beams;
θ0: a wedge angle;
ne: the refractive index of the ordinary beam; and
no: the refractive index of the extraordinary beam.
FIGS. 6A to 6C schematically illustrate a situation where light enters a plane diffraction grating to be diffracted. FIG. 6A three-dimensionally illustrates a situation where one light beam of the light split by a depolarization plate enters the plane diffraction grating to be diffracted. FIG. 6B two-dimensionally illustrates, along a zx plane, a situation where the light enters the plane diffraction grating to be diffracted. FIG. 6C two-dimensionally illustrates, along an yz plane, the situation where the light enters the plane diffraction grating to be diffracted.
As described above, the incident angle and the diffraction angle of the light can be resolved into zx components and yz components. That is, the incident angle of the light on the zx plane can be defined as α1, and the diffraction angle of the light on the zx plane as α2. On the yz plane, the incident angle of the light is the same as the diffraction angle of the light. Consequently, each of the angles can be defined as θ. In this case, the relationship between the incident angle to the plane diffraction grating and the diffraction angle by the plane diffraction grating is typically represented by the following expression (2):mλ=d cos θ(sin α1+sin α2)  (2)
where m: a diffraction order;
d: a grating constant;
λ: a wavelength;
θ: the angle between incident light and the depth direction of the groove;
α1: the incident angle of incident light to the diffraction grating; and
α2: the diffraction angle of diffracted light from the diffraction grating.
Returning to FIG. 7 in view of the above, the relationship between the incident angle to the plane diffraction grating 5 and the diffraction angle by the plane diffraction grating 5 is also represented by the expression (2).